Emerging functions within the enzyme families of plant alkaloid biosynthesis

The alkaloids are a major class of nitrogen-containing plant specialized metabolites that function as both toxic defense compounds and useful pharmaceuticals. Understanding the biosynthesis of these molecules offers a glimpse into the impressive diversity of metabolic chemistry found within plants and also enables the metabolic engineering of valuable alkaloids in other biological systems. Within the last decade, there have been major advances in our knowledge of alkaloid metabolism within multiple plant lineages, and this has allowed for the complete, or near complete elucidation of several alkaloid biosynthetic pathways. This work has been instrumental for establishing the chemical logic by which alkaloids are synthesized in plants and has also highlighted enzymatic functions that were not previously recognized or identified within plant metabolism. In this review, we highlight notable examples of unexpected or otherwise noteworthy enzyme-catalyzed reactions that have recently been identified within alkaloid biosynthesis. Our goal is to emphasize two major areas: 1) non-canonical reactions catalyzed by enzyme families with otherwise well-known roles in alkaloid biosynthesis, and 2) enzyme families that have emerged as having a broader role in alkaloid metabolism than previously appreciated. Beyond compiling these examples, we also discuss mechanistic details of novel enzyme classes that may be useful for future identification and study of non-canonical enzyme activity in alkaloid biosynthesis. Collectively, the recent discovery of several previously unidentified enzyme activities in alkaloid biosynthesis indicates that there is still much to uncover about plant specialized metabolism. Thus, the continued study of alkaloid biosynthesis will not only provide us with the underlying enzymes necessary to engineer production of valuable molecules with medicinal potential, but will also yield foundational insight on the chemistry that is possible in the plant kingdom.